Current-sensing circuit



Jan. 9, 1968 w. A. CARTER 3,363,144

CURRENT-SENS ING CIRCUIT Filed March 2l, 1967 1N VENTOR. W/z /f4/v/ 4. Cie/Ye United States Patent O M 3,363,144 CURRENT-SENSING ClRCUlT William A. Carter, Devon, Pa., assigner to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Mar. 21, 1967, Ser. No. 624,859 5 Claims. (6l. 317-33) ABSTRACT 0F THE DSCLSURE A circuit for applying a high energy pulse to a load circuit responsive to a particular instantaneous current magnitude in a circuit being monitored. A rst circuit driven by a low voltage contains a measuring circuit for monitoring current and, when the current exceeds some value, fires a controlled rectifier to permit the discharge of a series capacitor through a pulse transformer primary winding. The pulse transformer secondary winding fires a spark gap in a second circuit driven by a high Voltage to permit discharge of a high voltage capacitor through the load.

This application is a continuation-impart application of copending application Ser No. 447,636, led April 13, 1965, entitled Current-Sensing Circuit, assigned to the assignee of the present invention, now Patent No. 3,315,130.

This invention relates to a novel current-sensing circuit of the type shown in copending aplication Ser. No. 447,- 636, now Patent No. 3,315,130, referred to above, and more particularly relates to the use of a high and low voltage section for such circuits wherein the high voltage circuit drives the main capacitor and load while the loa voltage portion drives the control circuitry and control capacitor.

Current-sensing devices are generally well known to the art, and used, for example, for the operation of circuit protective devices, and which generate tripping signals for circuit protective devices when the current in the circuit being protected is subjected to fault conditions. Such dcvices typically operate responsive to rate of change of current in the circuit being protected, and give rise to time delays between the time that fault conditions occur and the time that an operating impulse is delivered to the circuit interrupter operating system.

The principle of the present invention is to provide a novel control circuit which responds to actual current magnitude as contrasted to rate of change of current whereby an output operating signal is delivered when current magnitude exceeds some predetermined value wherein the high energy portion of the circuit is driven from a high voltage source, while the controlling portion of the circuit is driven by a relatively low voltage source.

Accordingly, a primary object of this invention is to provide a fault-sensing system for ultra high speed protective devices in which only the main power portion of the circuit is driven by a high voltage source.

Another object of this invention is to provide a novel protective circuit for generating an output signal to an interrupter-type device responsive to a fault in the circuit being protected without any time delay which uses low control voltages.

Another object of this invention is to provide a novel static protective circuit having the characteristics of a relay which uses both high and low voltage supply circuits.

3,363,144 Patented Jan, 9, 1968 ICC These and other objects of this invention will become apparent from the drawing which shows a circuit diagram of the present invention for driving the impulse coil of an ultra high speed circuit interrupter device.

Referring now to the drawing, I have illustrated therein a novel control circuit which has a minimum of components, yet maintaining extremely high reliability for the fault-sensing device.

The drawing schematically illustrates a main conductor 1) which is a portion of a major circuit to be protected by the operation of the contacts of an ultra high speed circuit interrupter 11 connected in series in the line. The circuit interrupter 11 is of the type described in U.S, Patent 2,916,579 wherein the contacts thereof are operated responsive to the energization of an impulse coil schematically illustrated by the labeled circle 12 connected to the contacts 11 by the dotted line in the drawing. As will be seen more fully hereinafter, however, the impulse coil 12 could be replaced by any other electrically energiZable component which is to be energized responsive to the occurrence of predetermined current magnitudes in the line 10. The output terminal to which coil 12 is attached may be provided wih a shunt resistor having a resistance of about ohms. This resistor protects power components from danger should the impulse coil not be connected or should be open circuited.

A current transformer 12a is suitably connected to the line 10, and the secondary thereof is loaded with a low ohmic resistance burden comprising the resistor 13. The voltage appearing across resistor 13 need not be large, and may be less than 100 volts. Thus, the current transformer can be made small without sacricing accuracy. Moreover, the resistor 13 is non-inductive so that the voltage appearing across its terminals will be a replica of the current in the primary conductor 1l).

The terminals of resistor 13 are then connected to the A-C terminals of the single phase bridge connected rectier 14. The output of bridge 14 is then connected across a 25,000 ohm potentiometer 15 whose wiper arm 15a is connected to the base electrode of transistor 17 which can be of the type 2N697 through a 25K resistor 121 and a diode 122 which may be type IN3194.

The bottom of potentiometer 15 and its wiper arm receive capacitor 123 which may be a 0.1 microfarad capacitor. The components 121, 122 and 123 are not disclosed in the above noted copending application, with capacitor 123 acting as a noise suppressor while components 121 and 122 protect transistor 17.

The base emitter circuit of transistor 17 is then connected in series with a 20 volt Zener diode 18 whereby the Zener diode 18 will prevent conduction in the base emitter circuit of transistor 17 until the voltage drop across the left-hand side of potentiometer 15 exceeds the accurately calibrated value of 20 volts of the Zener diode 18. Note that other voltage values could be used for the Zener diode, depending upon the desired application of the circuit.

A thirty volt Zener diode 124, not shown in the above noted application, is connected, as shown, to serve as a clipper for transistor 17 and accurately calibrated Zener diode 18.

The transistor 17 is then coupled to a second silicon controlled switch 19 which may be of the type 3N83 through the 27K resistor 20 and the 5.1K resistor 21.

A ripple suppression circuit comprising a 9.1K resistor for the circuit, this circuit not shown in the above noted application. The output circuit of the transistor 19 contains a diode 127 which may be of the type INS 194 and is then connected through the 1K resistor 22 to the bottorn of Zener diode 18. A 20K shunt resistor 129 is connected in the trigger circuit of silicon controlled switch 19 as shown. Silicon controlled switch 19 is further connected in closed series relation with the gate electrode of controlled rectifier 23, the primary winding 24 of a pulse transformer 25, and an 11K resistor 26a. The controlled rectifier 23 may be of the type 2N1774.

As an important feature of the present application, a relatively low voltage sup-ply system is provided to drive the foregoing circuitry and includes a transformer 13() which is connected to a suitable A-C source and to rectifier 131. The positive terminal of rectifier 131 is then connected to the top of capacitor 26 and the right-hand side of resistor 26a, and has a potential of about 150 volts with respect Vto ground. The potential at the left of resistor 26a is about 67 volts with respect to ground which is used to drive the various biasing voltages for transistors 17 and silicon controlled switch l. Note that 150 volts is available for charging capacitor 2t?.

A spark gap 31 is then provided which has main electrodes y32 and 33 and a trigger electrode 34.

The secondary winding 35 of pulse transformer 25 is then connected through 25K resistor 132 across the trigger electrode 34 and electrode 32 and through impulse coil 12, whereupon a voltage across trigger electrode 34 and main electrode 32 will cause a small spark which will initiate the firing of the main electrodes 32 and 33. The electrodes 32 and 33 are then connected in series with impulse coil 12 and a Lttl-microfarad capacitor 35. Thus, when the spark gap 31 operates, the capacitor 35 will discharge through impulse coil 12, thereby to operate contacts 11. Capacitor 35 is charged to about 2500 volts through a relatively high voltage source comprising a standard AsC source connected to step up transformer 132 and series rectifiers 133 and 134 and series resistor 136 which may be a 5K resistor. As an important feature of the invention, this high voltage source is independent of the low voltage control source. A low voltage protective relay coil 155) controls protective contact 151 which is in series with resistor 137.

The circuit operates in the following manner:

The main current flowing through the main bus or conductor will produce a fractional voltage across the precalirbrated potentiometer through the current transformer 12 and rectifier 14. When this voltage which is proportional to current magnitude in conductor 10 eX- ceeds the voltage drop across the Zener diode 18, the transistor 17 conducts, resulting in conduction of silicon controlled switch 19. This conduction will then change the voltage on the gate of controlled rectifier 23, thus firing the controlled rectifier 23 and permitting the discharge of capacitor 26 through the primary winding 24 of pulse transformer 25. The discharge of capacitor 26 produces a high frequency pulse on winding 24, which high frequency pulse is transformed into the secondary winding 3S of pulse transformer 25. The high voltage pulse across winding 35 is then applied to the trigger electrode 34, thereby causing a break-down of the gap between main electrodes 32 and 33.

Thus, the spark gap 31 acts as a switch which connects the large capacitor 35 which is charged from the high voltage source, through the impulse coil 12, thereby providing the large energy required to initiate the protective operation caused by impulse coil 12 and its associated components 11, as described in U.S. Patent 2,916,579.

Nlote that spark gaps such as spark gap 31 require a voltage pulse of considerable magnitude in order to be fired reliably. The pulse transformer will produce a high frequency pulse of several kilovolts with only a slight delay in the order of l0 to 50 microseconds. This pulse is capable of consistently firing spark gap 31, thereby discharging capacitor 35 into the impulse coil 12 without resorting to a mechanical switching device which would be very slow.

Accordingly, the contacts 11 will be operated without any substantial time delay after the measurement of a current magnitude in conductor 1f) which exceeds some predetermined value. Note that the circuit operation is wholly independent of rate of change of current.

Although this invention has been described with respect to its preferred embodiments, it should be understood that many variations and modifications will now be obvious to those skilled in the art, and it is preferred, therefore, that the scope of the invention be limited not by the specific disclosure herein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege or property is claimed are dened as follows:

1. In a current-sensing circuit for energizing a device at a predetermined current'in a conductor; a pulse transformer having a primary and secondary winding, a first and second capacitor, a controlled rectifier, a spark gap having first and second spaced main electrodes and a trigger electrode, first circuit means connected to said conductor having an output DC voltage proportional to the instantaneous current magnitude of said conductor, and .second circuit means including transistor means and a Zener diode; said first circuit means connected to said second circuit means with its said output D-C voltage reverse biasing said Zener diode of said second circuit means through said transistor means; said second circuit means connected to the gate electrode of said controlled rectifier and firing said controlled rectifier responsive to reverse conduction of said Zener diode; said first capacitor connected in series with the cathodeanode electrodes of said controlled rectifier and said primary winding of said pulse transformer; said secondary Winding connected in series with said trigger electrode and one of said main electrodes of said spark gap; said second capacitor connected in series with said main electrodes of said spark gap and said load, whereby when said current in said conductor exceeds a predetermined current, said Zener diode becomes conductive to re said controlled rectifier, thereby discharging said rst capacitor through said secondary winding to cause said spark gap to fire, thereby to discharge said second capacitor through said load device; the improvement which comprises a first and second relatively high and relatively low voltage sources, respectively; said relatively high voltage source connected across said second capacitor to normally charge said second capacitor to a high voltage; said relatively low voltage source connected across said first capacitor to normally charge said first capacitor to a relatively low voltage; said transistor means having terminals; said relatively low voltage source connected to said terminals to provide biasing voltage for said terminals.

2. The combination as set forth in claim 1 which includes a second Zener diode connected in parallel with said first circuit; said second Zener diode having a higher breakover voltage than said Zener diode.

3. The device substantially as set forth in claim 1 wherein said first circuit includes a current transformer `connected to said conductor and a rectifier having A-C and D-C terminals; said current transformer having a secondary winding connected to said A-C terminals of said rectifier; said D-C terminals of said rectifier connected to said Zener diode.

4. The device substantially as set forth in claim 2 wherein said first circuit includes a current transformer connected to said conductor, a rectifier having A-C and ID-C terminals, and a transistor having base, emitter and collector terminals; said current transformer having a secondary winding; said secondary winding connected to said A-C terminals, said D-C terminals connected in series 5 6 with said zener diode and said base and emitter electrodes; References Cited said emitter and collector electrodes connected to said sec- UNITED STATES PATENTS ond circuit means.

S. The device substantially as set forth in claim l 2840766 6/1958 Wouk 31,7*16 wherein said conductor is connected in series with circuit 5 9262017 7/1966 Ashenden et al' u 31.7 33 breaker contact means; said load device connected to said (l) circuit breaker Contact means to operate said circuit breaker Contact means to a deenergized position responsive x 1 l to energization of said load device by discharge of said MILTON 0' HIRSHFIELD Pnmmy Exammer second capacitor therethrough. J. D. TRAMMELL, Assistant Examiner. 

